Honestly, I could be wrong but I think that most people want to know "How powerful the Wii is" and the important question is "as compared to what?" or "in what context?" ...
The Gamecube was an amazingly powerful system back in the day, being able to produce games with 15 Million polygons per second with 18 MB of texture data (the texture data was compressed to 2MB in the Flippers on chip memory); in the context of 480p this works out to be (roughly) between 0.7 and 1.4 polygons per pixel with 5 texels per pixel. On top of all of this the Gamecube had a TEV unit which enabled it to produce similar effects to what a pixel or vertex shader could produce on the XBox or PC.
If you look at it from a theoritical performance perspective the Gamecube should have been able to produce graphics that were far better than could be displayed by 480p when it was released. The problem is that we don't live in a theoritical world and between bottlenecks, tradeoffs and lack of optimization the Gamecube never achieved its theoritical promise.
Now, what we know about the Wii is that it is an enhanced version of the Gamecube's architecture. I think that it is a fairly safe assumption to think that Nintendo's goal with the Wii was to eliminate the bottlenecks in the Gamecube, improve real world performance, and enable even poor developers to produce graphics which match or exceed the output capabilities of 480p.
In comparison to the Gamecube the Wii is actually a pretty big step up, and in the context of a console that is focused on displaying graphics at 480p the Wii is very powerful.
